Performance & Delaying Detonation - Pit Stop

Delaying DetonationQ It seems there is a conflict between gasoline quality and high compression, which is where we performance people like to make horsepower. It would help if I understood what detonation is and things that affect it.
Greg Peters
Chatsworth, CA

A As the air fuel charge is ignited, the burning charge and unburned charge in the combustion space experience a rise in temperature and pressure. Toward the end of the burn, the remaining charge may reach its self-ignition temperature. This usually occurs in a location remote from the spark plug. Because the conditions of this remaining charge allow spontaneous combustion, there is no flame front associated with the sudden application of high pressure to all surfaces and components included in the combustion process. This phenomenon is generally defined as detonation.

In the combustion process, there is a brief period between the initiation of spark and actual burn. By definition, this "delay period" can be used to determine the effects of changes to an engine that affect cylinder pressure. As a rule, lengthening the delay period helps to reduce the tendency to detonate.

Of the variables that have an effect on detonation, some of the following may be of interest. Increased compression ratio raises the temperature and pressure associated with the "end charges" before the normal burn can reach them. The octane rating and the quality of the fuel can affect the delay period and, therefore, detonation.

Increasing intake manifold (or inlet) pressure tends to promote detonation because the delay period will likely be shortened. Mixture density is also higher, and this increases flame speed. Higher charge temperature also tends to shorten the delay period and increase the tendency to detonate.

You may find it interesting that combustion chamber design can affect detonation as well as its compatibility with fuel type and compression ratio. Again, locating the spark plug near the center of the chamber, not allowing the end-charges to burn near a hot exhaust valve, and using small chambers that promote short flame travel distances are all options to control detonation.

Brake Break-inQ I have a '90 5.0L Mustang that I sometimes use for parking lot slalom racing. Some of my friends are switching to high-performance brake pads. I thought I read something in HOT ROD that said to break in new pads. If this is true, how should I do it?
Chris Brock
Belmont, MA

2/3

A Different pad materials may require different break-in methods. Keep in mind that fresh rotors should also be broken-in, for reasons similar to the pads.

With fresh pads, make numerous but gentle braking stops to gradually bring friction surfaces up to temperature. Even if you experience some degree of brake fade during these steps, don't worry about it. Let the brakes cool for a few minutes, and repeat the process a couple of times. As for bedding-in the pads, it's then safe to make some serious stops. The objective is to prevent premature overheating of the pads, causing permanent damage to the friction properties of the material.

For rotors, follow the same guidelines, unless recommended otherwise by the manufacturer. Hard braking with new rotors that haven't been properly seasoned can cause metallurgical changes in the material, possibly leading to mal-performing brakes.

Cut to the CoilQ I'm trying to restore a '66 Chevy El Camino. I'm doing some work that includes minor lowering of the rear end. I don't know if the rear coil springs are stock or not, but it sits too high. How many coils should I cut out, and can I do this with a cutting torch like I've been told?
Eddie Newberry
Tulsa, OK

3/3

A Don't use a cutting torch! This can change the heat-treat of the spring, potentially causing premature failure, early sag, or an unacceptable ride height. If you cut the springs at all, use a cut-off wheel or comparable tool, and make sure you deburr any sharp edges. Make sure the springs aren't wound from tapered wire. If they are, don't cut 'em.

Your best bet would probably be to check out springs that are either stock replacements or designed to provide the ride height you want. Trimming coils out of a stock unit can be akin to cutting off one leg of a chair. Surely you know where this can lead.